Transcript EvolutionofPopulations209

Industrial Melanism and
Microevolution
Hierarchical Classification
Gene Flow
additions to and/or subtractions from
a population resulting in the movement
of fertile individuals or gametes
Gene Flow and Human Evolution
Increasing migration of people
throughout the
world has contributed
to an increase in
gene flow
Mutation and Sexual Recombination
produce the variation that makes
evolution possible
Genetic Drift occurs by chance when only
certain members of a population reproduce
and pass on their genes
Genetic Drift
Bottleneck
Effect: a sudden
change in the
environment drastically
reduces the size of the
population
Cheetah
Northern Elephant Seals
Polydactylism in the Amish Population
Founder
Effect:
migration of
a small subgroup
of the population
Founder Effect in Amish
Ellis-van Creveld Syndrome
Causes dwarfism and polydactyly
The evolution of fruit fly (Drosophila)
species on the Hawaiian archipelago
(Founder Effect)
• Natural Selection is the primary
mechanism of adaptive evolution
• Out of all the factors that can affect
a gene pool only natural selection is
likely to adapt a population to its
environment
Mapping Malaria and the Sickle-Cell Allele
This is a good
example of
heterozygote
advantage.
Modes of Selection
Types of Selection
• Most traits are polygenic - variations in
the trait result in a bell-shaped curve
• Three types of selection occur:
1) Directional Selection – the curve
shifts in one direction ex:
resistance to antibiotics by
bacteria
Directional Selection
Evolution of the Horse over 50 million yrs
Hyracotherium
American Museum
of Natural History
Orohippus
Note the
toes!
Directional selection for
beak size in a Galápagos
population of the medium
ground finch
Types of Selection
–(2) Stabilizing Selection
•Ex - when human babies with
low or high birth weight are
less likely to survive
Stabilizing Selection
Cepaea Disruptive Selection
Snails
(3) Disruptive Selection
The curve has two peaks; dark shells
appear in most forested areas whereas
light-banded shells appear in areas of
low lying vegetation
Ex – When Cepaea snails vary because a
wide geographic range causes selection
to vary
Disruptive or Diversifying Selection
Small-billed birds feed on soft seeds; largebilled birds feed on hard seeds (Blackbellied Seed Crackers – Cameroon, Africa)
The Two-Fold Disadvantage of Sex
Why Natural Selection Cannot Fashion
Perfect Organisms
1) Evolution is limited by historical
constraints.
2) Adaptations are often compromises.
3) Chance and natural selection interact.
4) Selection can only edit existing variations.
Natural selection can affect the
distribution of phenotypes in
three ways. They are:
_______________ selection
_______________ selection
and
_______________ selection.
A small population of organisms is
suddenly cut off from the others
in the population. This is known as
the _____________ effect.
A small group of organisms migrates from
one area to another. There is not a wide
variation in the gene pool. This is known
as the ___________ effect.